1. Field of the Invention
The present invention relates to an improved process for reacting gaseous raw materials by contacting same with a catalyst and an improved vertical cylindrical reaction vessel for conducting such reaction.
2. Description of the Invention
Conversion reaction accomplished by contacting a gaseous starting material with a solid catalyst under an appropriate pressure is now utilized for synthesis of ammonia, synthesis of methanol, methanation and other various purposes. In many cases, this conversion reaction is an exothermic reaction conducted under a pressure higher than atmospheric pressure. Accordingly, the temperature of the gas and catalyst is extremely elevated by the heat of reaction, resulting in degradation of the catalyst performance and reduction of the concentration of the intended product because of the chemical equilibrium. Therefore, such excessive elevation of the temperature of the gas and catalyst should be avoided, and many contrivances have heretofore been made for attaining this object.
A most popular method for removing the heat of reaction is to utilize the heat of reaction for preheating a starting gas to be supplied to the catalyst layer. More specifically, according to this method, heat exchange is conducted between a high temperature gas present in or leaving from the catalyst layer and a low temperature gas being introduced into the catalyst layer to elevate the temperature of the gas introduced into the catalyst layer to a level necessary for initiating the reaction. Ordinarily, the heat exchange is carried out under substantially the same pressure as the reaction pressure, and a catalyst and a gas-gas heat exchanger are disposed in one reaction vessel.
In this method for preheating the introduced gas by utilizing the heat of reaction, the quantity of the heat of reaction is larger than the quantity of the heat necessary for preheating, and in the catalyst layer, the temperature on the effluent side is much higher than the temperature on the introduction side. Therefore, no satisfactory advantage can be attained with respect to the above-mentioned catalyst activity or chemical equilibrium. Moreover, this method is defective in that another apparatus is necessary for recovering the excess heat reaction sufficiently and the level of the recovered heat energy is reduced.
Another method of removing the heat of reaction, which is frequently adopted, is to remove the heat of reaction by evaporating a liquid under an appropriate pressure. As the liquid to be evaporated, water is most preferred from the practical viewpoint, though Dowtherm or a hydrocarbon mixture having an appropriate boiling point can also be used.
In this method, it is essential that the liquid be evaporated under a pressure providing an appropriate boiling point lower than the catalyst temperature in order to prevent excessive elevation of the temperature of the catalyst, and the heat of reaction is removed by the evaporation of the cooling liquid. Ordinarily, the pressure of the cooling liquid is lower than the pressure of the gas, and a reaction vessel having a structure as shown in FIG. 1 illustrating the vertical section thereof is used.
Referring to FIG. 1, upper tube sheet 2 and lower tube sheet 3 are gas-tightly fixed to a pressure shell 1, and a large number of tubes 5 are gas-tightly fixed to these tube sheets 2 and 3. A net 4 is disposed below the tube sheet 3 to support a catalyst, and a catalyst layer 6 is packed inside each of tubes 5. A gas compressed to a reaction pressure and preheated to a appropriate temperature is introduced from a gas inlet 7, passed through the catalyst layer 6 and fed to the next step from a gas outlet 8. While the gas is passed through the catalyst layer, the conversion reaction takes place, and the heat generated by the reaction is transferred through the catalyst layer 6 and the wall of each of the tubes 5 to a cooling liquid introduced from a cooling liquid inlet 9 to a space between the pressure shell 1 and the tube sheets 2 and 3 outside the tubes 5 and the heat of reaction is removed by boiling and evaporation of the cooling liquid. The cooling liquid is discharged from an outlet 10 in the form of a vapor or a liquid-vapor mixture, and the heat possessed by the vapor or liquid-vapor mixture is used for attaining an intended purpose, for example, driving a turbine to compress the starting gas. The reaction vessel of this cooling liquid evaporation type is advantageous over the above-mentioned reaction vessel of the gas-gas heat exchange type with respect to prevention of excessive elevation of the temperature of the catalyst and effective utilization of the heat of reaction.
However, from the viewpoint of recently increased importance of saving of energy and necessity of increasing the size of a single reactor, new problems arise in the reaction vessel of the cooling liquid evaporation type.
The first problem is that in order to increase the degree of effective utilization of the heat of reaction, a higher vapor temperature and a higher vapor pressure are desired. For example, when the generated vapor is steam and it is introduced into a turbine and converted to a mechanical energy, steam held at 100 Kg/cm.sup.2 G and 480.degree. C. is expanded to 40 Kg/cm.sup.2 G, thereby an energy of about 50 KWH per ton of steam is recovered. However, if the pressure of generated steam is 40 Kg/cm.sup.2 G, the energy corresponding to this steam pressure is not recovered. The structure shown in FIG. 1 is not suitable for a large scale reaction vessel. When the high pressure of generated steam is desired.
The second problem is that the reaction pressure is reduced. For example, in the manufacture of ammonia or methanol, a pressure of 100 to 300 Kg/cm.sup.2 G is ordinarily adopted for the conversion reaction. In this case, it is necessary to compress the raw gas from a pressure level of 20 to 40 Kg/cm.sup.2 G at the gas generation step to a level of 100 to 300 Kg/cm.sup.2 G. If the pressure for the conversion reaction is reduced below 100 Kg/cm.sup.2 G, the power necessary for this compression can be remarkably reduced. For the reason of chemical equilibrium, reduction of the reaction pressure is possible if there is available a catalyst having a high activity at a low temperature. Furthermore, it is known that the lower are the temperature and pressure for the conversion reaction, the larger is the quantity of the heat generated by the reaction, and that degradation of the activity by excessive elevation of the temperature is conspicuous in a catalyst exerting the catalytic activity at a low temperature. Accordingly, in order to solve this second problem, it is necessary to increase the sectional area of the gas passage for flowing of a low-pressure and large-volume raw material gas and completely control the temperature inside the catalyst layer. However, this can hardly be attained by the structure of the reaction vessel comprising a catalyst layer disposed in a tube as shown in FIG. 1.
The third problem arises with increase of the size of a single reactor. More specifically, in the reaction vessel shown in FIG. 1, in order to prevent leakage between the cooling medium and reaction effluent gas differing in the pressure, it is necessary to fix gas-tightly the tubes 5 to the tube sheets 2 and 3 and also fix gas-tightly the tube sheets 2 and 3 to the pressure shell 1 by welding or other means. However, since the temperature of the tubes 5 is different from the temperature of the pressure shell 1, the vertical length between the tube sheets 2 and 3 is changed because of the difference of thermal expansion during the operation. Even if a metal material having a thermal expansion coefficient substantially equal to that of a material of the pressure shell 1 is used for the tube 5 as disclosed in Japanese patent application Laid-Open Specification No. 49707/73, a large thermal stress is produced in the tube sheets 2 and 3. This thermal stress is increased as the diameter and length of the reactor are increased. Therefore, designing of a large-size reactor becomes difficult.
Still another problem is that if the pressure on the cooling medium side is elevated and the pressure on the effluent side is lowered from the viewpoint of saving of energy as pointed out hereinbefore, it is necessary to increase the thickness in each of the pressure shell 1, tube sheets 2 and 3 and tubes 5, resulting in a serious economical disadvantage in case of a large-size reaction vessel.